A large proportion of electrical power generated in the world is produced by systems and processes that transfer thermal power from a heat source to water to produce steam and produce electrical power from the steam. Such systems and processes include fossil fuel power plants including coal-fired power plants, oil-fired power plants, and natural gas-fired power plants; nuclear power plants; geothermal power plants; waste incineration power plants; and solar thermal electric plants. Together these systems and processes for producing electrical power from thermal power account for about 80% of electrical power production worldwide, where coal-fired power plants produce about 41%, natural gas fired power plants produce about 20%, nuclear power plants produce about 15%, and oil-fired power plants produce about 6% of the total world electric power.
Many of these systems and processes produce significant quantities of carbon dioxide as an undesirable by-product emission. In particular, fossil fuel power plants such as coal-fired power plants, oil-fired power plants, and natural gas-fired power plants generate substantial quantities of carbon dioxide as a by-product of combustion. Typically, the carbon dioxide produced by these plants is emitted into the atmosphere, contributing to the increase of carbon dioxide in the atmosphere.
Electrical power production by steam-based thermal to electrical power systems and processes is relatively inefficient. Typically, steam-based thermal to electrical power systems generate 13-58% electrical power as a percent of the heating value of the fuel consumed. Standard fossil fuel plants utilizing sub-critical steam as a heat transfer agent for producing electrical power typically generate from 36-40% electrical power as a percent of the heating value of the fuel consumed. Nuclear power plants tend to be less efficient than standard fossil fuel plants, generating electrical power at 30-32% of the heating value of the fuel consumed due to the lower operating temperatures, and consequently lower steam pressures, at which nuclear reactors are run relative to fossil fuel-fired plants.
Substantial efforts have been made to increase the efficiency of steam-based thermal to electrical power systems and processes by even a few percentage points of the heating value of the fuel consumed. For example, power plants have been designed that utilize thermal power to produce supercritical steam (steam having a temperature of at least 374° C. and a pressure of at least 22.15 MPa), thereby improving the efficiency of electrical power generation to about 40-45% of the heating value of the fuel consumed. Currently, efforts are being made to utilize thermal power to produce ultra supercritical steam (steam having a temperature of at least 374° C. and a pressure of at least 30 MPa), thereby improving the efficiency of electrical power generation to about 48% of the heating value of the fuel consumed. Combined cycle gas turbine plants with integtrated heat recovery steam generator systems have been developed that can generate electrical power in an amount up to 53-58% of the heating value of a natural gas fuel consumed to produce the electrical power.
The Kalina cycle, as illustrated in U.S. Pat. Nos. 4,489,563 and 4,732,005, has been introduced as a method for increasing the efficiency of electrical power generation relative to a purely water/steam based thermal to electrical power system. Electrical power generating systems utilizing the Kalina cycle replace a water/steam based working fluid with an aqueous ammonia working fluid, where the ammonia/water concentrations of the aqueous ammonia working fluid can be varied so that the aqueous ammonia working fluid may attain a temperature close to the temperature of the heat source. The Kalina cycle is particularly useful for capturing thermal power from heat sources having relatively low heating value, for example, low grade and conventional geothermal power and industrial waste heat. The Kalina cycle may increase the efficiency of electrical power generation from a heat source having a relatively low heating value by 5-10% of the heating value of the fuel consumed relative to a purely water/steam based thermal to electrical power system.
Other working fluids are being developed to replace the water/steam working fluid that is utilized as a heat transfer agent in most thermal to electrical power systems. For example, a thermal to electrical power system is being developed by Saena corporation that utilizes supercritical carbon dioxide as a working fluid instead of water/steam. Specialized equipment and systems and seals, however, are required to handle supercritical carbon dioxide at temperatures above 400° C., temperatures that are typical in thermal to electrical power systems. These specialized systems, equipment, and seals may be impractical in a large scale commercial electrical power plant capable of generating over 100 MW of electrical power.
Journal article Pilot Plant Demonstrates Steam-Ammonia Binary Cycles, Modern Power Systems, vol. 5, no. 2, 71-75, March (1985) provides a binary cycle system utilizing a water/steam working fluid as a heat transfer agent in a first cycle and a working fluid other than water having a lower condensing temperature than water and a low freezing temperature, ammonia for example, as a heat transfer agent in a second cycle, where the second cycle is a “bottoming cycle” that receives its heat from low energy steam produced in the first cycle. Water is heated by a heat source (e.g. nuclear fission or an oil-fired boiler) to produce steam that is expanded in an expander to produce electrical power and low energy steam. Heat is transferred from the low energy steam to the second working fluid in a boiler/condenser to produce a vapor stream from the second working fluid that is expanded in a second expander to produce additional electrical power and to produce an expanded vapor that is subsequently condensed in a condenser by exposure to an ambient air flow. The bottoming cycle permits the use of air cooling to condense the second working fluid rather than evaporative cooling requiring water, permitting a power plant to be sited in locations where there is no large supply of water for evaporative cooling. Substantial power, however, is lost by air cooling the second working fluid because the latent heat of condensation of the second working fluid is rejected to the atmosphere in the air warmed by heat exchange with the second working fluid to condense the second working fluid.
Japanese patent application JP2000145408 A2 also discloses a binary cycle system utilizing a top cycle having steam as a working fluid and a bottoming cycle utilizing a working fluid having a boiling point lower than water such as benzene, pentane, and ammonia. The expanded vapor produced by vaporizing the working fluid of the bottoming cycle and expanding the resulting vapor to produce electrical power is condensed by air cooling or water cooling of the expanded vapor in a condenser. The efficiency of electrical power production is disclosed as being improved from 11% to 17-18% relative to a single cycle steam based system. Again, substantial power is lost by air cooling or water cooling the bottoming cycle working fluid because the latent heat of condensation of the bottoming cycle working fluid is rejected to the atmosphere in the air, or lost to the cooling water, warmed by heat exchange with the bottoming cycle working fluid.
There is a need for practical steam-based thermal to electrical power generating systems and processes that produce substantial amounts of electrical power per unit fuel, particularly steam-based thermal to electrical power generating systems that produce significantly more electrical power per unit fuel than currently utilized electrical power generating processes and systems. Further, there is a need for thermal based fossil fuel-fired electrical power generating systems and processes that produce little or no carbon dioxide emissions.